The present disclosure relates to a matrix image sensor comprising a plurality of individual detection structures associated with respective pixels.
The dynamic range of such a sensor is decisive in regard to preserving all the information of a scene to be observed. In the visible spectrum, the spatio-temporal distribution of the luminance may be greater than 120 dB, thus greatly exceeding the dynamic range available on a conventional image sensor.
These use a linear photoelectric conversion law whose operating dynamic range is limited at one and the same time by the maximum amplitude of the video signal and the level of the noise at the output of the sensor.
EP 1 271 930 discloses a sensor whose resolution may be modified. There is no operation in solar cell mode of the photodiodes.
Numerous solutions are known for increasing the operating dynamic range of an image sensor.
Patent EP 1 354 360 discloses for example a CMOS individual detection structure associated with a pixel having a logarithmic response and good image quality. This individual detection structure nonetheless exhibits two structural defects: heavy electrical consumption when a large number of these structures are assembled within a matrix to produce a sensor and the appearance of black columns on the image formed on the sensor in the case of strong point-wise and local illumination of the scene to be observed.
An individual detection structure 1ij according to patent EP 1 354 360 has been represented in FIG. 1. This structure 1ij comprises a photodiode 3ij in photovoltaic mode (that is to say corresponding to a solar cell mode where the photodiode generates a voltage under illumination) read by an amplifier stage 4ij with infinite continuous impedance comprising two MOS transistors 41ij and 42ij, the output of this amplifier stage 4ij being connected to a reading bus 7j via a selection MOS transistor 6ij. This structure also comprises a switch 8ij making it possible, when it is closed, to short-circuit the photodiode 3ij, as described in patent EP 1 354 360.
In order to guarantee the operating stability of the photodiode in photovoltaic mode, the amplifier stage 4ij must be kept permanently operational, dynamic powering of this amplifier stage creating non-acceptable switching noise on the photodiode 3ij which is in a state of high impedance.
When detection structures 1ij according to FIG. 1 are assembled according to a matrix whose size is for example greater than 768×576 pixels, this being the standard TV resolution in Europe, the electrical consumption required to permanently bias the amplifier stage 4ij of each individual detection structure 1ij of the matrix can attain a value having prejudicial consequences in regard to the operation of the image sensor produced with the aid of such a matrix.
As represented in FIG. 2, a matrix comprising a large number of detection structures 1 can exhibit a stray capacitance 71 on the reading bus 7j of significant value, and this may require a larger bias current in the amplifier stage 4ij of each individual detection structure 1ij. This bias current is for example of the order of 1 μA, which for a matrix of 1000×1000 pixels corresponds to a global current of 1 A. Such electrical consumption may pose difficulties with the design of the sensor and cause overheating of the microchip in which the sensor is integrated, thus increasing the dark current in the photodiodes and degrading the photoelectric performance of the sensor.
Another drawback of an image sensor produced with structures according to FIG. 1 is manifested by the risk of black columns appearing in the image formed on this sensor when the latter acquires an image of a scene in which very intense point sources of light are present, these black columns corresponding to pixels whose signal cannot be read.
As represented in 3, when a point light source is projected onto a subset of detection structures 1ij according to FIG. 1, the light source causes the appearance of a significant photoelectric current Ipp in the drain and the source of the selection transistor 6ij between the output of the amplifier stage 4ij and the reading bus, this current Ipp corresponding to the appearance of stray photodiodes 61 between the drain of the transistor 6ij and earth and between the source of the transistor 6ij and earth. When this photoelectric current Ipp is sufficiently large with respect to the bias current of the amplifier stage 4ij, no reading of any signal relating to the individual detection structure 1ij can be performed on the reading bus 7j, thus causing the appearance of a black patch on the image formed. Knowing that a reading bus 7j is shared by the whole set of detection structures associated with the pixels of one and the same column, this creates a black column in the image formed on the sensor.
Furthermore, the individual detection structure disclosed by patent EP 1 354 360 may also not be optimal in terms of compactness since it requires that the transistors 41ij and 42ij of the amplifier stage 4ij on the one hand, and the selection transistor 6ij be of different type, the transistors 41ij and 42ij of the amplifier stage 4ij being for example P-channel MOS transistors and the selection transistor 6ij being an N-channel MOS transistor. With a structure according to FIG. 1, the output signal from the amplifier stage 4ij indeed exhibits too low a voltage for a P-channel MOS transistor, which might be used as selection transistor, to be able to be turned back on.
Moreover, the individual detection structure disclosed by patent EP 1 354 360 does not possess any means for storing an image. Now, progressive reading on a matrix comprising individual detection structures such as these creates a temporal shift between the start of reading and the end of reading, and this may cause deformations of objects in motion when the latter are observed by a sensor comprising such a matrix.